The mayor made a clear commitment to Londoners in his election manifesto to personally take charge of the Metropolitan Police Authority (MPA). Those of us with experience of the MPA knew immediately that he had made a promise he could never live up to. But Boris made crime the number one issue of the election and his chairing the MPA was the number one pledge in the crime section of his manifesto.

In Boris Johnson's manifesto, the following commitment was first on the list for crime: "Provide strong leadership: by taking responsibility and chairing the Metropolitan Police Authority and using my influence to tear up red tape and needless form-filling, so we can get more police out on the streets."

As expected, he has now gone back on his word, realising that being both mayor and chair of the MPA is just too much for one person to do properly when both are steep learning experiences. It was an ill-thought-out promise, and one that showed his lack of knowledge about what being mayor and chair of the MPA would involve.

It's obvious he has worked hard at being mayor, although his politics-by-press-release style infuriates me because it makes him so much less accountable than Ken Livingstone, but he never did much to absorb the role of MPA chair. So, quite honestly, I don't believe it will make that much difference to police accountability or the lives of Londoners.

Johnson's deputy, Kit Malthouse, has been doing the hard work of running the MPA show behind the scenes while the mayor just turned up to chair the meetings or meet the commissioner occasionally. To suggest, as Malthouse has, that there was "heavy lifting" in the first two years, which Boris had to do and did do, is farcical.

Meanwhile, most of the responses to major controversy, such as the G20 demonstrations and the Worboys rape case, came from individual members of the Metropolitan Police Authority rather than by Johnson or Malthouse. It is always worth remembering that the mayor doesn't have a guaranteed majority on the MPA and Malthouse will need to maintain the mayor's good track record of keeping a cross-party consensus to make the authority work.

Here at City Hall we've noticed that Johnson has been stretched to cope with the job of mayor. It's a big job, especially for someone who has never run anything of this kind. And this is despite the fact that the overwhelming majority of the projects which Boris is "delivering"– Crossrail, the East London Line extension, Tube upgrade and even the ideas on cycle hire and cycling superhighways – were inherited from the previous mayor.

He has dumped large numbers of new projects by declaring them to be unfunded and left a vacuum in Transport for London's plans for coping with congestion on the roads and overcrowding on public transport. London is a growing city and if the mayor hopes to fill the gap between the growing demand for transport and the supply of new capacity with more people cycling, then he must reverse his cut to cycle lanes in outer London and make the roads safer.

Boris has also stood down as chair of the London Waste and Recycling Board amid criticisms that he is not delivering on environmental action. The mayor has failed to keep any of his own launch dates for environmental action plans. This may have something to do with around three-quarters of Livingstone's excellent environment team leaving City Hall as Boris drastically cut the number of staff and delayed key actions such as banning polluting white vans from London. The public consultation on tackling air pollution will reach consultation stage when Boris is celebrating two years as mayor. We have yet to see a first draft of his plans to tackle climate change and meet the ambitious target of a 60% cut by 2025 which he (also) inherited from Livingstone.

Livingstone raised council taxes during most of his years as mayor in order to increase police numbers by over 8,000 and create a system of safer neighbourhood teams in every area of London. Johnson has frozen the council tax precept for the second year running and he has cut millions of pounds from the Met police budget. The Met face difficult times ahead, with budgets being cut in all areas. The chair of the MPA needs to take the time to understand this complex organisation to provide effective leadership. Johnson has not really been involved from the beginning and perhaps feels it is time to stop pretending.

One last interesting factor is that Johnson's admitting he can't do both roles blows a hole in Tory plans for elected mayors who control the police. Other mayors, in other cities, do manage both roles, but I'd be willing to bet they are experienced politicians who have worked hard to know their remit, not amateurs doing an apprenticeship on the job. If he had to give up something, why not give up his writing commitment at the Telegraph instead of the MPA chair, something that Londoners have every right to expect from him?

What do the Tories think of Boris' move? Did he clear it with anyone? Did he assess what it would do to the Tory policy? Perhaps it would have been best to tough it out, stick at the MPA meetings, keep his promise. He doesn't want to be thought lightweight, does he?